ACROSS

5. Greenhouse gases.

6. Deforestation.

8. Landfill.

9. Climate change.

10. Microplastics.

11. Environment.

DOWN

1. Atmosphere.

2. Fossil fuels.

3. Emissions.

4. Solar power.

5. Global warming.

7. Recycling.

Letter in the green boxes:
O U L I L P T N
(Em chỉ thấy có 8 ô xanh nhưng lại có tới 9 chữ cái lận ạ...)

The unscrambled word is:
P O L L U T I O N

My own crossword clue for this word:
The introduction of harmful materials into the environment.